Invalid block parameters syntax: incomplete parameters list,

Error message

Invalid block parameters syntax: incomplete parameters list, expecting "|" but the tag was closed prematurely

What it means

After at least one block param identifier was parsed, the parser expected the closing `|`, but instead hit `>` or `/` (tag close). This means the parameters list was left unclosed before the tag ended.

Source

Thrown at packages/@glimmer/syntax/lib/parser/tokenizer-event-handlers.ts:430

              throw generateSyntaxError(
                `Invalid block parameters syntax: expecting the tag to be closed with ">" or "/>" after parameters list`,
                as.start.until(loc)
              );
            },
          };
        } else if (next === '|') {
          if (element.params.length === 0) {
            // Following Handlebars and treat empty block params a syntax error
            throw generateSyntaxError(
              `Invalid block parameters syntax: empty parameters list, expecting at least one identifier`,
              as.start.until(this.offset().move(1))
            );
          } else {
            state = { state: 'AfterEndPipe' };
            this.tokenizer.consume();
          }
        } else if (next === '>' || next === '/') {
          throw generateSyntaxError(
            `Invalid block parameters syntax: incomplete parameters list, expecting "|" but the tag was closed prematurely`,
            as.start.until(this.offset().move(1))
          );
        } else {
          // slurp up anything else into the name, validate later
          state = {
            state: 'BlockParamName',
            name: next,
            start: this.offset(),
          };
          this.tokenizer.consume();
        }
      },

      BlockParamName: (next: string) => {
        assert(state.state === 'BlockParamName', 'bug in block params parser');

        if (next === '') {

View on GitHub (pinned to 26f97246a8)

Solutions

Add the closing pipe before the tag close: `<Foo as |a|>`
Check each opened `|` has a matching closing `|`

Example fix

// before
<Foo as |a>
// after
<Foo as |a|>

Defensive patterns

Strategy: validation

Validate before calling

// Each `as |` must have a matching closing `|` before `>` or `/`
if (/<[^>]*\bas\s\|[^|>]*[>\/]/.test(template)) {
  throw new Error('Block params list missing closing "|" before tag close');
}

Prevention

Always pair the opening pipe with a closing pipe before `>`
Use prettier/glimmer formatting to normalize block params

When it happens

Trigger: parsePossibleBlockParams (via appendToAttributeName) when the next character is `>` or `/` while state is inside the parameters list — e.g. `<Foo as |a>` or `<Foo as |a/>`.

Common situations: Forgetting the closing pipe when writing `as |a`; deleting the second pipe during edits; mistaking the tag close for the list terminator.

Related errors

AI-assisted analysis of emberjs/ember.js@26f97246a8 (2026-09-01). Data as JSON: /api/errors/fd4f5ae99a0ddcc1. Report an issue: GitHub.